In broad definition, a tube may be any hollow object that has at least a first open end and second open end. There are many kinds of products in the market based on a tube.
Relative to drinking straws, and since its advent in time immemorial, the drinking straw has always been a tube with a first open end and second open end and of circular cross section. Prior to the existence of manufactured drinking straws, the circular shape was as a result of drinking straws being little more than a length of hollow rye grass or the like.
In 1888, Marvin Stone invented the first manufactured drinking straw by spiral winding paper around a circular shaft and gluing it together.
In current day production, the material of choice is plastic, and most typically polypropylene, and the production method of choice is extrusion. With the process of extrusion, many cross-sectional forms can be generated, however drinking straws have typically remained of circular cross section.
With the ever increasing need to achieve higher production rates in order to minimise unit production costs, one of the main production bottlenecks is the grooving of drinking straws to create one or more flexible sections. Currently extruders can produce in excess of 1,200 drinking straws per minute while the fastest grooving machines typically can only operate at speeds in the order of 600 drinking straws per minute. This upper limit speed of drinking straw grooving is principally dependent on the number of grooves being formed down the length of the drinking straw.
There are a number of existing methods relative to forming corrugations or grooves in a drinking straw.
U.S. Pat. Nos. 2,631,645, 2,985,077 and 3,242,828 teach a range of methods of corrugating or circumferentially grooving sections of a drinking straw by means of various blades or dies rotating around the exterior surface of the drinking straw with the purpose of deforming the surface into corrugations or grooves.
U.S. Pat. No. 3,339,004 teaches a method of corrugating or circumferentially grooving a section of a drinking straw by means of inserting rods into each end of a drinking straw, clamping the drinking straw on to the rods, then moving the rods towards each other thereby causing the drinking straw section entrapped between the rods to collapse and form into pleats.
U.S. Pat. No. 3,751,541 teaches a method of corrugating or circumferentially grooving a section of a drinking straw by means of an external mould that moves at the same speed as the drinking straw tube as it exits the extrusion die head and whereby a vacuum is applied to the outside of the drinking straw to draw the drinking straw into the mould where it assumes the external configuration of the mould cavity.
U.S. Pat. No. 4,613,474 teaches a method of corrugating or circumferentially grooving a section of a drinking straw by inserting up inside the drinking straw a grooved mandrel and then rotating the drinking straw and grooved mandrel over external and similarly grooved bars or dies, whereby the drinking straw is deformed to assume the externally grooved configuration of the mandrel.
U.S. Pat. No. 6,685,103 teaches a method of longitudinally grooving a section of a drinking straw by means of inserting a shaped mandrel up inside the drinking straw and deforming the drinking straw to assume the external configuration of the mandrel by means of external rollers.
There are also a number of existing methods relative to forming corrugations or grooves in tubes for applications other than drinking straws.
U.S. Pat. No. 3,021,245 teaches a method of corrugating or circumferentially grooving a section of a tube by applying material to the surface of a shaped mandrel, applying a vacuum to draw the material to conform to the external configuration of the mandrel, using an external mould if necessary to assist with forming the external shape of the tube, then pressurising inside the mandrel in order to expand the tube so it can be removed from the mandrel.
U.S. Pat. No. 3,949,045 teaches a method of forming a pipe coupling by means of a pipe being softened by heat then placed inside an external mould while an expandable mandrel is inserted inside the pipe, and whereby on the mechanical expansion of the mandrel, the pipe is deformed into the external configuration of the mould cavity to form a pipe coupling.
U.S. Pat. Nos. 5,529,743, 6,170,535 and 6,508,275 teach variations of a method of corrugating or circumferentially grooving sections of a tube by means of extruding a parison of thermoplastic material, placing the still hot parison into a mould, then blow moulding the parison such that it expands outwardly under pressure to assume the external configuration of the mould cavity.
The major drawbacks in the existing methods become apparent when one seeks to achieve the maximum possible extent of surface profiling, contouring, shaping, corrugating or circumferential grooving whilst at the same time achieve the maximum possible production speed.
While there are a number of methods that deform the surface of a drinking straw into corrugations or grooves through the use of various mandrels, dies and rotating blades, the method that has proven to yield the highest production rate with the highest degree of consistency is the method as taught by U.S. Pat. No. 4,613,474.
The method currently adopted in the marketplace as the standard is based on the insertion of a grooved mandrel up inside the drinking straw followed by the rolling of the drinking straw and the grooved mandrel between external and similarly grooved dies. As already stated, such machines can operate at production speeds in the order of 600 drinking straws per minute.
With the existing method however, once the grooves in the drinking straw have been formed through the deformation process, difficulty arises in maintaining the straightness of the flexible drinking straw.
This difficulty is accentuated as either production speed is increased or the number of grooves down the length of the drinking straw is increased and standard practice has shown that production speeds in the order of 600 drinking straws per minute are typically only achievable when in the order of 10 to 12 grooves are formed at one time. Any increase in the number of grooves to improve drinking straw flexibility results in a necessary reduction in production speed in order to maintain straightness.
As a production process, blow moulding allows any reasonable combination of surface shapes to be formed without using an internal mandrel that conforms to the inner surface of the product being produced. The existing methods for the production of corrugated bottles, tubes and the like are typically based on the blow moulding method of extruding a parison, and then while the parison is still hot, placing the parison in an external mould and using pressure to conform the parison to the configuration of the mould cavity.
While as a process this maximises the range of shapes that can be formed, the process of extruding parisons is slow compared to that of current day extruders. A typical parison extruder produces in the order of 4 to 6 parisons per minute.
The slow production rate of this style of machinery was also an issue relative to the requirement of high speed blow moulding of plastic bottles, particularly for the beverage sector. To resolve this production speed bottleneck, a new method of blow moulding was introduced as taught by U.S. Pat. No. 3,969,060.
U.S. Pat. No. 3,969,060 teaches a method of blow moulding bottles based on the deformation of a tubular slug of thermoplastic material. The slug, or preform as it is known, is produced in an injection-moulding process then at a later time it is inserted into a stretch blow-moulding machine whereby the preform is heated then expanded outwardly under pressure to assume the configuration of the cavity of an external mould.
Irrespective of which blow moulding technique is used, whether blow moulding from a parison extruded directly on a blow-moulding machine or from an externally produced preform, all of these processes are typically based on a closed-loop system, that is where there is only one open end such that the product can be expanded or ‘blown’ outwardly under pressure, as with air being pumped in to expand or ‘blow up’ a balloon.
In order to achieve the highest production speed possible for any form of tube, the primary production method should preferably be via high-speed extrusion, however irrespective of the method of tube construction or the tube profile, whether circular or any other cross-sectional form, a tube is open-loop, that is it has a first open end and a second open end. A tube therefore is not inherently suitable for known blow moulding processes.
U.S. Pat. No. 3,079,637 teaches a method of bottle production based on the re-heating of an extruded tube by the placing of the tube on a mandrel, applying heat to soften the tube, placing the heated tube and mandrel in a mould whereby the mould pinches closed one end of the tube and then conforms the heated tube to the mould cavity through the application of blow-moulding techniques to produce a bottle with only one open end.
U.S. Pat. No. 3,149,373 teaches the same method as U.S. Pat. No. 3,079,637 with the addition of teaching a plurality of tubes and mandrels blow-moulded at the same time, each individual tube and mandrel producing only one bottle in each mould cycle and each bottle having only one open end.
US Pat. No. 3,449,792 teaches a similar method to U.S. Pat. Nos. 3,079,637 and 3,149,373. The apparatus in this specification uses a separate member to grip the unsupported end of the tube in an attempt to symmetrically centre the unsupported end of the tube after the heating process, due to frozen-in stresses from the extrusion process that cause the tube to assume a ‘banana’ shape during re-heating of the tube on the mandrel in a significant percentage of cases.
The above three patents teach only the production of a single bottle out of a tube. The mandrel as taught is a wire-frame arrangement that does not extend through the full length of the tube and does not fully support the tube as the tube is re-heated.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art.
The object of at least preferred embodiments of the present invention is to overcome some of the disadvantages with the existing apparatuses or methods, or to at least provide a useful alternative choice.